Signaling



J. N. WHITAKER SIGNALING Filed oct. 14, 1939 Aug. 26, 19141,

INVENTOR. Jff .N. WH/TAKER ATTORNEY.

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Patented Aug. 26, 1941 A'UNITED [STATES PATENT ofFFIcE;

J ames N. Whitaker, Weehawken, N. J assignor to -Radio Corporation ofAmerica, a corporation of Delaware Appliation otober 14,1953, serial No.2.99421 vio claims. (c1. 25e-2o) This invention relates toa radiosignaling systern and particularly to the switching of a circuit fromone signal to another, depending upon which signal `is the stronger.More specifically, it relates to a vacuum tube and gas filled tubenetwork wherein a means is provided for automatically switching from astandard or primary signalfto an alternative or secondary signal at suchtimes asthe primaryl signal maybe below a usable level, but only if thelevel of the secondary signal is of a usable value. Y

'This invention, although especially useful in connection with thesub-carrier frequency modulation system described in the U. S. patentapplication of James E. Smith et al., Serial No. 270,332, filed April27, 1939, is not restricted thereto.

In the above mentioned application Serial No. 270,332, the use of theswitch was shown in block diagram form in Figure 2, and a schematicdiagram of the switch Was shown in Figure 5. The operation `of theswitch was described in the body of the specification. This Vswitch wasarranged to transfer the control of the system from the primary orfundamental input to the secondary or harmonic input at any time whenthe lfundamental input was reduced in level to a predetermined value,and to again restore the control to the fundamental input when the levelof said fundamental input was restored to its original usable level.This switching took place automatically and irrespective of the level ofthe signal in the secondary (or harmonic) channel.

"Experience has proven that the energy present in the secondary (orharmonic) channel was not always of a usable value when the signal inthe fundamentalchannel had dropped below the predetermined level.Therefore, when the switch transferred the control to the harmonicchannel, the only signal present was often radio circuit noise, and theprime purpose of the switch (that of filling in a missing portion ofthe' received intelligence) was defeated.

, The purpose of the present inventiony is to improve the operation ofthe switch Vand `to provide a differential operation in order that theswitching may only take place after t'he fundamental signal has beenreduced in amplitude below a predetermined level, and then onlyproviding the signal in the harmonic channel is of a greater amplitudethan the signal remaining inthe fundamental channel; f

Having thus described the purpose of the present invention, reference ismade to the figure in the drawing and the following description.v

nal intoy the input 2 of amplifier tube 3. The circuit of this amplifieris conventional and will be understood from the drawing withoutI furtherdescription. The output circuit 4 introducesthe signal into full waverectifier 5. AThe positive terminal 6 of ,this rectifier isconnected toone end of resistance l Yand the center tap of transformer 4 (whichrepresents the negative terminal of the rectifier system), is connectedtothe other end of the resistor. A smoothing condenser 8is"ccnnected'acrossthese teri'nin'als.vr o

Control lines Ia'introduce the harmonic signal into the input 2a o-famplifiertube 3d;YY The circuit of this amplifier, like that ofamplifiers is conventional and merits no further description. The outputcircuit 4c introduces the signal vinto full wave rectier 5a. Theypositive ter-v minal 5a. of this rectifier is connected to posi-tiveterminalA 15, of the fundamental rectifier and therefore also to one endofV resistance V'1. The center tap of transformer da (which'representsthe 'negative terminal of this secondv rectifier system), is connectedto`one 'end of'resistance The other end of resistance 9 connects to oneend ofresistancel.y A'smoothing'c'ondenser (la. is ccnnected acrossresistor 9, and an adjustfaible tap on this resistor is grounded.l

The elements thus far described constitute the didveren'tial arrangementused inconnection with the electronic switch. This'diier'e'ntialarrangement asshown inthe drawing is' set apart from the reniainiei`ofthe switch by a dashed line t'o'assist in the' explanation, whichcovers the operation of the entire system. A y,

The negativeend of resistance 9v is connected @TW-,gh @n a'QPlQPf-'lte1e5t0` vl!) 'll grid 'H 0f triode The positive end yof resistor 9 isconnected thnoughf an vaulipropriate 'negative grid biasing ource I3grid resistorv I4 to grid il gas ,triode I6. These gas triodes are also`as' Vthyratrons,` grid glow tubes,et,c`.`, andV usually'haye a potentialdrop of approximately `15 volts across theirv anodes and cathodes whenconducting'.VVV The anodes I9 and?? of gas triodes v,IZ and A,lli areconnected @garner andro the/positive terminal 41 of' a suitable powersource,Y of which the negative terminal is indicated as 42. Terminal ,42is also connected to ground. The cathodes ll-'l and I8 y of the gastriodes are connected together through connected `through resistance 22'to one end of resistance g3, which end is also connected to the cathddesof push-pullamplifijer tubes 24and 25.

j Control HDSS l .intfoduceihs fundamental 51g.- 55 Tile other .engl oflresstarwe 23 is grounded and CII resistance 39 and to the cathodes ofpush-pull;

amplifier tubes 3| and 32. The opposite end of resistor 3D is connectedto ground and also to the center of the secondary windingof transformer33, which is fed by input lines 34. The opposite ends of the secondaryof transformer 33 are connected to the grids of amplifier tubes 3l and32. The anodes of these tubes are connected to the outside ends of theprimary winding of transformer 35. The center tap of this winding isconnected to cathode Il of gas triode I 6.

The secondary of transformer 35 is connected between the grid ofamplifier tube 36 and ground. The secondary of transformer 28 isconnected between the grid of amplifier tube 31 and ground. The anodesof tubes 36 and 31 connect to opposite ends of the primary Winding ofoutput transformer 38, the center tap of which is connected to positiveterminal 4I of the power supply. The anodes of amplifiers 36 and 31might also be connected in parallel and to one side of the primarywinding of transformer 38, if desired. In that case, the opposite sideof the winding would then -be connected to terminal 4I of the powersupply. The cathodes of these tubes are connected in parallel throughbiasing resistor 39 to ground. A by-pass :condenser 40 is connectedacross resistor 39. The secondary of transformer 38 is connected to theutilization circuit which may be a limiter as shown in Figure 2 of Smithet al., application Serial No. 270,332, filed April 27, 1939.

The action of the complete differentially operated electronic switch isdescribed as follows. Two separate operations are performed in theoperation of the switch. First, the control of the switching by the twosignals through a differential arrangement and, second, the actualtransfer between two signals by means of the electronic switch proper.The two signals which are switched may be the saine two signals that actdifferentially to operate the switch.

The differentiation between the two signals is a function independent ofthe operation of the switch. The operation of the switch, however, isdependent upon the differential between the two controlling signals.Therefore, it is found convenient and less confusing to first describethe action of the differential arrangement and then to describe theoperation of the electronic switch, resulting from the differentialcurrentsrderved from the two signals.

The action of the fundamental and harmonic signal amplifiers andrectifiers incorporating elements I to 6 and Ia to 6a inclusive andrespectively is conventional and will be understood by reference to thecircuit diagram -without further description.

Let us assume for the moment that there is a fundamental signal input toI. This signal is amplified and rectified and produces an IR drop acrossresistor 1. The end of I which connects to elements 6 and 6a ofrectifiers 5 and 5a will be positive with respect to the opposite end oithis resistor. Let us say that the Potential cl-"- veloped acrossresistor 'I by the fundamental signal is X volts. Let us also assumethat a signal is also present at Ia of the harmonic channel. Thisharmonic signal will be amplified, but rectifier a cannot pass currentunless the harmonic signal reaches a higher value than the fundamentalsignal. This is due to the biasing action on rectifier 5a due to the IRdrop across Vresistor 1 caused by the fundamental signal. If

the level of the fundamental signal is reduced, or

if the signal level in the harmonic channel is lncreased so that thesignal as rectified by 5a is greater than X volts, current will fiowthrough resistors 9 and I from rectifier 5a. This current flow willcause a potential to appear across resistor 'I which will be greaterthan X volts and rectifier 5 will cease to pass signal currents for thesame reason as previously described for rectifier 5a. Thus, it will beunderstood that the only rectifier which may pass current is the onereceiving the stronger signal.

In actual practice the fundamental signal is always considerablystronger than the harmonic signal except under conditions where theR.-F. carrier fades out and permits the two sidebands to beat togetherand generate a strong second harmonic. For this reason it is apparentthat rectifier 5a will operate only during these abnormal conditions.

The combined elements of the fundamental channel operate to produce an1R drop across resistor 'I only, and `only acts to prevent the operationof rectifier 5a.

The elements of the harmonic channel operate to produce an IR dropacross both resistors 7 and 9. Resistor 9 also forms a part of theelectronic switch. Therefore, the signal applied to the harmonic channelnot only acts to block the signal output of the fundamental channel, butalso operates the electronic switch.

I-Iaving thus described the operation of the signal differentiatingmeans, I will proceed to describe the action of the electronic switchwith reference to the schematic diagram shown in the figure of thedrawing.

We will again assume that the fundamental signal is usable; andrectifier 5a is blocked off by a drop of X volts across resistor 1. Gridbias I3 places a sufiicient negative bias on the grid I5 of gas triodeI6 to prevent conduction in this tube. No bias is applied to grid II ofgas triode I2, which is now conducting. Current is flowing through thistube and resistors 29 and 39 to ground. The entire power supply voltage(minus the l5 volt tube drop) will now appear across these tworesistors. Cathode I8 will be at a positive potential with respect toground. Cathode I8 also connects to the center tap of transformer 28. Bymeans of this connection, a positive potential is applied to the platesof amplifier tubes 24 and 25 through the transformer. The IR drop acrossresistor 23 due to the plate current of these amplifiers will provide anormal cathode bias for class A operation. Therefore, the fundamental(or doubled fundamental) signal will be amplified and passed on to theutilization circuit.

Let us now assume the .second condition. The fundamental signal has beenreduced until the IR drop across resistor 'I is substantially lower Asthe input to the I3 begins to conduct.

creased rectifier 5a will 'pass current, through resistors 9 and 1.. .Anegative .bias is applied through .resistor HlA to grid Il .of gastriode I2 because-of the IR. drop Aacross resistor 9. Likewise, apositive potential will be applied to .grid 45 of gas triode t6 throughbias. I3 andY resistor I4. (This is due tothe division .of the potentialappearing across resistor 9 by the variable tap, which is grounded.) Asthis positive potential is increased, bias lwis overcome, and gas triode(This conduction isoften spoken of as firing To say such a tube is firedmeans that the Vconduction is started.)

When gas triode i6 .is fired, it raises the potential at the terminal.of .condenser :2.4 by the amount of drop across resistors V22 and 2.3.The potential of cathode .i8 is momentarily raised above the potentialof anode 2E). The. ycur-rent through tube i2 is thus .interrupted andthe tube ceases to ldraw current, or is extinguished t The time constant.of .condenser 2l is such that `it vcannot .discharge sufficiently fastto lower the cathode .to 'its operating voltage .before de.- ionizat'lonoccurs in `tube 1.2. Therefore, grid H kresumes control. .Since grid His now made negative by the signal drop in resistance 9, tube `l2 cannotrestrike at .this .time after vde-ionization takes place. YCi'mclenser2l thereafter dis.- charges thr-ough resistors 2.9 and 36 and rechargesin the opposite direction.

When tube I6 was fired and tube t2 wasextinguished, plate voltage wasremoved from arnplifier tubes 24 and 2.5. Blocking bias wassimultanecusly removed from tubes .3| and 32. Blocking bias was alsoapplied to tubes 24 and 25 due to the IR dropacross resistor .23. fore,amplifiers .24 and 25 became inoperative. Likewise, amplifiers .3i and32 became operative, and the harmonic or secondary signal was passed onto the output..

When thefundamental signal again returns vto normal, or if the harmonicsignal drops below a predetermined value, the bias .on grid Il of the.tube i2 is reduced and this' tube fires, and the opposite of the aboveaction occurs, returning the .switch to the first operating condition.

Although :primarily intended for use in connection with sub-carrierfrequency modulation systems, this invention should not .be limitedthereto.

For instance, any two signals vmay be .switched by any other twosignals. Additional elements may be added to provide a vmeans forcontrolling the selection of any one of `three or more signals fro-masmany inputs, etc.

Also, it may be found desirable to use such a device to switch .between.tivo signals by a .prearranged tone signaling device, such as, for

instance, a time clock.

The system may .also vbe. adapted to double frequency keying wherein onesignal of a given frequency would be transmittedfor marking and anothersignal of a .dierent frequency'transmitt'ed for spacing. n

It may conceivably be ,used in a secrecy. sys- .tem where part of theelements of a letter were to be transmitted on one frequency and a vparton the other.

I also do not wish to be limited to gas triodes, as any gas lled tubecontaining two or more elements could conceivably be used. This appliesparticularly to gas filled pentode tubes.

To obtain an .overall ,picture of mypresent invention it may be restatedbriefly as follows: g As v.ezvrplained in the .copending application lofThere- James E. Smith et jal., Serial blo-270,332, filed April V27,v1939,.variab1e. light impulsesy picked up by a scanning device vareemployed to produce corresponding electrical currents. .These .cur-

rents are employed to amplitude modulate an Y alternating current `ofconstant frequency. The amplitude modulated currents are rectified andthe rectified currents or Waves are employed to yfrequency modulate asub-.carrier alternating current wave. This frequency modulatedsubi-carrier, which may be of an audible or super-audible frequency, inturn is employed to amplitude modulate a high frequency.v or radiovfrequency carrier wave. The carrierV wave is vtransmitted to the.receiver and picked up, as shown in the circuit diagram, by thereceiving antenna RA.

The picked up waves are fed into a superheterodyne receiver SI-IR whoseoutput is the fre.- quency modulated sub-carrier wave. AIn the .eventthat the carrier wave fades, the super-heterodyne receiver will producea second harmonic output effectively caused by the beating together .ofthe sidebands received by the receiving antenna RA.

When the carrier is present and the super-- heterodyne receiverreproduces vin its output .a strong fundamental frequency modulated.subcarrier, the filter FF passes the fundamental Asubcarrier with itsfrequency modulations on to a limiter L and also.. as .explainedpreviously, tothe transformer 2 through line l. The output of limiter Lis fed through a low pass filter LPF, then to a frequency doubler FD, asexplained in the copend-ing application referred to, to transformer 26through lines v2l.

As explained, when .the radio frequency carrier is strong and a strongfundamental subcarrier is produced, this `sub-carrier of doubledfrequency as .obtained from frequency' doubler FD 4is fed throughtransformer 26, amplifiers 24 and 25 and amplier 31 to the utilizationor output circuit .indicated at 38.

When the radio frequency carrier fades so .that a strong second harmonicof the sub-carrier is produced in the output of the super-heterodynereceiver SHR, this harmonic energy passes by way vof the band passyfilter BPF tothe harmonic transmission lines I a.. As explainedhereinabove, With the strong second harmonic and a weak fundamentalsub-carrier switching takes place so that the utilization circuit 38 issupplied with the harmonic energy through circuit la, conductors 34,transformer 33, amplifiers 3|, 32 and amplier 36. n f Y It should beclearly understood thatthe frequency modulated sub-carrier `'at thetransmitter shown in Figure 1 of the copending application of Smith, etal., referred to above, may be employed to frequency modulate the radiofrequency carrier generated at the transmitter. In that event, -thesuper-heterodyne receiver'r SHR should be Vof thetype adapted to receive`and translate the frequency modulatedy radio frequency waves into thefrequency modulated Ysub-carrier. Also, rif. desired at the transmitter,the frequency moclu lated sub-carrier may be Vemployed to 'phasemodulate the radio frequency carrier, in which event the receiver SHRlshould be a phase modulation receiver. Also, if desired at thetransmitter, the sub-carrier may be phase ymodulated and this phasemodulated sub-carrier `employed to phase modulate the radio frequencycarrier Wave, in which event, as 1before,'the `.receiving systern SHRshould be a phase modulation receiver. Also, .if a. phase *modulatedsub-carrier 'is employed yat the .transmitter .thisphase modulatedsub-carrier may be employed to amplitude modulate the radio frequencycarrier, in which event the receiver SHR should be a simple amplitudemodulation receiver.

When the phase modulated sub-carrier is employed, the second harmonic ofthe phase modulated sub-carrier which appears in the output circuit at38 may be translated by any suitable phase modulation translator. Thus,for example, the phase modulated sub-carrier may be fed to a detectorthrough a lter which passes only one sideband of the doubled sub-carrierand the doubled carrier frequency of the sub-carrier. The detectedoutput in this case would be a replica of the modulating envelopereferred to hereinbefore.

Having thus described my invention, what I claim is:

1. A receiving system for receiving a carrier modulated by a sub-carriercomprising means for deriving from the modulated carrier sub-carrierwave energy, means for filtering out fundamental sub-carrier energy fromthe derived energy, additional means for filtering harmonic energy fromthe derived sub-carrier energy, means for raising the filteredfundamental energy to a harmonic frequency, and means includingtrigger-acting discharge tubes for selectively controlling the transferof the raised energy or the directly derived harmonic frequency energyto a translating device, said ,selectively controlling means beingadapted to favor one of said energies when both are of substantiallyequal value.

2. In a receiving system for a fundamental carrier wave containing amodulation frequency, means providing separate transfer paths for thefundamental carrier wave per se and for a harmonic component of saidwave respectively, a translating device having an input circuit underalternative control of the energy in the two said paths, electronicswitching means including gaseous discharge tubes for selecting thetransfer path which is appropriate to the stronger of said fundamentaland harmonic waves, and means controlled by the stronger of the two saidwaves for firing the proper gaseous discharge tube to cause the pathselection.

3. In a receiving system the combination according to claim 2 andincluding means effective when one of said gaseous discharge tubes isfired for extinguishing the other of said tubes.

4. In a receiving system, the combination according to claim 2 andincluding a push-pull amplifier in each of said transfer paths, each ofsaid amplifiers having a grid bias control circuit under the influenceof the conductive and non-conductive states of said gaseous dischargetubes.

5. In a receiving system., the combination according to claim 2 andincluding a push-pull amplifier in each of said transfer paths, each ofsaid amplifiers having an output circuit the source of anode potentialfor which is supplied through the space discharge path of one of saidgaseous discharge tubes respectively.

6. The method of selecting one of two transfer circuits between a radioreceiver and a translating device, by means of two alternativelyconducting gaseous discharge tubes, which coniprises separatelyrectifying a modulated subcarrier wave component and a harmonicderivative of the side bands of the same sub-carrier wave, causing afirst one of said tubes to be conductive so long as the amplitude ofrectified energy from said sub-carrier wave component eX- ceeds apredetermined value, causing a second one of said tubes to becomeconductive and the first said .tube to be extinguished when theamplitude of said rectified energy falls below said predetermined value,and causing each said tube to control an appropriate one of saidtransfer circuits in such manner that only the stronger of the signalscarried by said sub-carrier wave and its harmonic derivativerespectively are passed to said translating device.

7. In a device for switching from one potential source to another, autilization circuit, a rst and a second gas tube, means whereby eachtube at times starts to conduct and substantially simultaneouslyquenches the other, a first and a second amplifier tube, each amplifiertube having its output circuit connected to said utilization circuit,the input circuit of one amplifier tube being connected to one of saidpotential sources and the input circuit of the other amplifier tubebeing connected to the other of said potential sources, means forapplying a positive potential to the plate of the first amplifier tubefrom the output circuit of the first gas tube While it is conducting,and means for applying a positive potential to the plate of the secondamplifier tube from the output circuit of the second gas tube While itis conducting.

8. In a device for switching from one potential source to another, autilization circuit, a first and a second gas tube, means whereby eachtube at times starts to conduct and substantially simultaneouslyquenches the other, a first and a second amplifier tube, each amplifiertube having its output circuit connected to said utilization circuit,the input circuit of one amplifier tube being connected to one of saidpotential sources and the input circuit of the other amplifier tubebeing connected to the other of said potential sources, means forapplying a positive potential to the plate of the first amplifier tubeand a negative blocking potential to the grid of the second amplifiertube from the output circuit of the first gas tube while it isconducting, and means for applying a positive potential to the plate ofthe second amplifier tube and a negative blocking potential to the gridof the first amplifier tube from the output circuit of the second gastube while it is conducting.

9. In a device for switching from one potential source to another, autilization circuit, a first and a second gas tube, means whereby eachtube at times starts to conduct and substantially simultaneouslyquenches the other, a first and a second amplifier tube, each amplifiertube having its output circuit connected to said utilization circuit,the input circuit of one amplifier tube being connected to one of saidpotential sources and the input circuit of the other amplifier tubebeing connected to the other of said potential sources, means forapplying a positive potential to the plate of the first amplifier tubeand a negative potential to the plate of the second amplifier tube fromthe output circuit of the first gas tube while it is conducting, andmeans for applying a positive potential to the plate of the secondamplifier tube and a negative potential to the plate of the firstamplifier tube from the output circuit of tlie second gas tube while itis conducting.

10. In a device for switching from one potential source to another, autilization circuit, a first and a second gas tube, means whereby eachtube at times starts to conduct and substantially simultaneouslyquenches the other, a first and a second amplifier tube, each amplifiertube having its output circuit connected to said utilization circuit,the input circuit of one ampliiier tube being connected to one of saidpotential sources and the input circuit of the other amplier tube beingconnected to the other of said potential sources, means for applying apositive potential to the plate of the first amplifier tube and anegative potential to both the plate and grid of the second amplifiertube from the output circuit of the iirst gas tube While it isconducting, and means for applying a positive potential to the plate ofthe second amplifier tube and a negative potential vto both the plateand the grid of the rst amplifier tube from the output circuit of thesecond gas tube while it is conducting.

JAMES N. WHITAKER.

